The present invention relates to a cutoff valve, particularly for the cutoff of high-temperature fluids.
Cutoff valves are known with a spherical plug shutter, also commonly termed ball valves, widely used in industry as process valves, generally composed of a valve body which defines internally a valve chamber endowed with an input opening and an output opening, coaxial to each other. The valve chamber houses a substantially spherical shutter element traversed by a passage with a cross section generally equal to the cross section of the openings and supported by two pivots: a lower one, located in the valve body, and an upper one protruding from the valve body, which is used to operate the shutter.
By applying a deflecting torque to the upper pivot, the shutter rotates through 90.degree. and the abovesaid passage can be positioned coaxially with the openings so as to connect them to each other without creating any resistance to the fluid. By rotating the shutter through 90.degree. in the opposite direction, the passage is positioned perpendicular to the openings and the surface of the shutter element exerts a sealing action against two seats arranged at the openings, cutting off the passage of the fluid.
The seats are in metal, and the surface thereof which comes in contact with the shutter is shaped as a spherical segment, with a curvature equal to that of the shutter; however, the sealing action is effected by an elastomer ring located in the seats.
The metal surfaces ensure a rough sealing action in case of emergency, that is to say when e.g. due to a fire, the elastomer ring is burned.
The seats can be pressed against the shutter by a series of springs; the pressure of the fluid adds a further pressing action, so that the specific pressure required for effecting the sealing action is generated between the shutter and the elastomer ring.
These kinds of valve have the disadvantage, due to the fact that the sealing action is performed with an elastomer ring, that they cannot be used for high-temperature fluids and/or with some kinds of fluid which would quickly compromise the integrity of the elastomer ring.
On the other hand, in the case of a seal without the elastomer ring, that is to say in the case of a metal sealing action, the forces generated by ordinary springs are not sufficient to ensure a satisfactory sealing action and cannot be increased beyond a certain limit without compromising the integrity of the seats and/or of the shutter.
To eliminate this disadvantage, valves have been manufactured with a floating shutter, that is to say with a shutter which is no longer guided on pivots but rests against one of the seats, generally against the seat of the output opening.
In this case, the pushing action generated by the pressure of the fluid against the spherical surface of the shutter, is superimposed to the pushing action generated by the springs, when the valve is closed onto the seat. When the pressure reaches a sufficiently high value, the sealing action is ensured.
This kind of valve is, however, not devoid of disadvantages.
At low pressure, the sealing action is very uncertain, while at high pressure the friction of the two metal surfaces in contact, under heavy load, may damage them and cause them to seize, particularly in severe operating conditions, that is to say in the presence of soiled fluids with solid particles or for incipient corrosion due to the fluid.
The elimination of this disadvantage has already been obtained in the known art by dividing in two the motion of the shutter: a 90.degree. rotation (from open to closed) without contact with the seat and a subsequent translation towards the seat. This motion is obtained by shaping appropriately the upper pivot and allowing a preset clearance between the lower pivot and the body of the valve.
In turn, these valves have the disadvantage of being more difficult to operate in the case of automatic driving devices, because of the need to provide two kinds of motion.